1. Field of the Invention
This invention relates in general to surface inspection systems, and in particular, to a high speed scanning system for detecting anomalies of an object with high precision by using one or more stereo camera sets.
2. Description of the Related Art
Even the human eye with its remarkable ability to capture huge amounts of optical information cannot make measurements with precision and accuracy sufficient for quality control of parts requiring highly precise tolerances. Failure to achieve proper measurements can result in a wide range of problems. For example, failure to maintain dimensional tolerances for parts in an automobile engine may result in problems ranging from decreased system life to increase probability of failure under out of the ordinary conditions. Even where later system testing at a manufacturing facility can detect problems resulting from failure to meet dimensional tolerances, unacceptably high rejection rates on account of defective systems can result.
Moreover, even when human eye is capable of performing a required inspection, such factors as fatigue, lighting, distraction and so forth make human inspection unreliable.
Today, in an attempt to minimize those applications where human optical inspection is employed, to avoid the sometimes unpredictable problems caused by fatigue, distraction and other factors, industry has turned increasingly toward the implementation of computerized inspection systems. With such systems, however, the resolution of the optical inspection apparatus is far below that of the human eye.
Currently, many electronic imaging cameras use two dimensional arrays of light sensitive elements, sometimes of the type known as charged coupled devices (CCD) as photodetectors. The purpose of these devices is to convert an optical image into a video image. There are many relatively low priced color and black and white array CCD detectors available for video imaging, but they produce low quality images, as alluded to above. More particularly, CCD detectors and other video imaging devices suffer from a relatively low pixel count. For example, CCD photodetector arrays have the ability to produce quality images with a resolution of approximately 2048 by 2048 pixels. However, these arrays are presently very expensive. Moreover, these very large arrays tend to have defects such as inoperative pixels, inoperative clusters of pixels, or inoperative lines of pixels. When very high quality images are required this type of electronic imaging system is not only very expensive. Such systems may not even be capable of performing a high-quality, high precision measurement.
Linear photodetectors cost much less than array detectors because they have far fewer pixels and thus have correspondingly much higher manufacturing yields. Linear photodetectors, obviously, however, are capable of imaging only one line of information in an image that any given point in time using the single line of photo sensitive devices which they have. Accordingly, the linear photodetector must therefore scan the entire image, line by line.
The same is achieved by using a mechanical scanning assembly for moving the linear photodetector across the image plane in the camera. Generally, systems of this type derived image data by 1) relying upon the precision of the steady state operation of the mechanical scanning assembly, 2) assuming identical transient translational movements during the initiation of a scan, and 3) assuming that translational movement is uniform over time. Such a system, while suitable for making high quality digital images for commercial photography, is inappropriate for use in making high precision measurements.
More particularly, mechanical irregularities in the scanning assembly make the generation of highly precise image information impossible, thus making the image data captured by such systems on suitable for the purpose of confirming dimensional tolerances in a precision manufacturing environment.
Accordingly, it would be advantageous to have a device that will capture a single image of a part with enough precision that accurate measurements can be made of the features of such a part.